Cascade impactors are devices used to segregate and quantify aerosol particle size distributions based upon aerodynamic and other properties. Some cascade impactors collect the aerosol particles on filter media. The majority and most common cascade impactors utilize metal plates as the collection surface. In some devices this metal collection surface is the top of the next collection stage. In others, it is a separately inserted collection plate. Nevertheless, in order to avoid bounce-off and blow-off of the particles from the collection surface, it is common to apply a coating such as grease or other sticky substance to trap and retain the particles.
Trapping and retention of the aerosol particles on the proper stage is paramount to determining the correct size distribution. But also important is the need to assure that the coating substance, while present at an amount effective to trap and retain the particles, does not interfere with the subsequent analysis. Interference can occur through physical and/or chemical means. Interference with chemical analyses will be related to the quantity or mass of the coating material on each stage in relationship to the mass of aerosol particles to be assayed.
Conventional methods for coating impactor collection plates have involved spraying them in ventilated fume hoods or on the counter top with grease from pressurized containers. Other methods involve dipping the plates into, or using a pipette to flood the tops of them with, a solution or suspension of the coating material and allowing the coating to dry to a sticky consistency before loading the plates into the impactor. These methods provide poor control over the total amount of coating material applied and thus no reliable means to avoid interference with the chemical assays.
Thus there is a need to assure efficient particle trapping with minimal assay interference by applying the coating materials only where they are needed; that is, only where the aerosol particles will be impacting the collection plates or surfaces. This area is referred to as the particle-collecting regions of the collection substrates, collection surfaces, or stages. These areas are distinct from the other regions of the substrates and other structural portions of the impactor such as the walls and orifice plates.